This invention relates, generally, to compositions and methods useful for sealing or plugging porous or fractured subterranean formations encountered during the drilling of vertical or horizontal earthen boreholes, or during the excavation of earthen trenches, while fluid is flowing into or out of the borehole or trench. The invention also generally relates to compositions and methods useful for sealing or plugging porous or fractured formations associated with any subterranean cavity, which may have fluids flowing into or out of it. More specifically the invention relates to compositions and methods involving coated mixtures of bentonite and water-swellable polymer which, when mixed with an aqueous fluid, result in a pumpable suspension useful for sealing or plugging porous or fractured subterranean formations.
Rotary drilling methods have long been used to drill vertical boreholes to reach subterranean aquifers, oil and gas reservoir zones and mineral deposits. In most of these boreholes a drilling fluid comprised of water, clay, especially bentonite, polymers, weighting materials and other additives is used to lubricate the drill pipe as it rotates in the bore, cool the drill head as it cuts through the formations and carry the soil and rock cuttings produced during the process back to the surface. If the circulating drilling fluid were to be rapidly lost to porous formations encountered during drilling, so that it could not be quickly and fully replaced, the integrity of the bore might be compromised. For this reason the development of a wide variety of lost circulation control agents has long been a priority in the drilling industry.
More recently vertical drilling techniques have been adapted for the drilling of relatively shallow, horizontal bores using a process known as horizontal directional drilling. The typical depth for most horizontal directional drill bores is less than 50 feet below the surface. This process is used to install such things as oil, gas, water and sewer pipelines, and power and telecommunications cables without the need for extensive surface excavations. The limited impact that this method has on streets, highways and other surface infrastructure makes it particularly useful in metropolitan areas.
The horizontal orientation of the borehole in this method results in a large amount of friction between the drill pipe and the borehole. This makes it impractical to rapidly rotate the drill pipe to power the drill head and enable it to bore the hole, as is typically done in vertical bores. As a result, alternate methods of drilling have been developed. The simplest and most often used of these methods relies on pumping a fluid under high pressure through the drill pipe and out through small openings in a xe2x80x9cduck-billxe2x80x9d shaped drill bit where it acts to hydraulically cut through the soil that the bore is penetrating. When this technique is employed it is standard practice for the drilling fluid to be composed of water, or water and bentonite clay. The drill pipe is then advanced through the soil as it is cut and flushed away by the hydraulic action of the drilling fluid at the drill bit. It is quite common in this type of drilling for all of the drilling fluid, and the drill cuttings that it contains, to be lost into the formation that is being drilled. This method is very effective in relatively soft, loose and unconsolidated soil, sand or gravel zones. It is not effective in hard rock situations, however.
When hard rock is encountered a drilling method is used that employs a rotating tri-cone bit similar to that used in most vertical drilling operations. In vertical drilling the rotation of the drill pipe also rotates the drill bit allowing the teeth on the cones to cut the formation into small pieces that can be flushed out of the hole. In horizontal directional drilling the friction of the drill pipe against the bore hole, especially along the bottom of the hole where the pipe usually lies, makes it impractical to rapidly rotate the drill pipe to power the drill bit. Instead, a xe2x80x9cmud motorxe2x80x9d located within the drill pipe, that is powered by the hydraulic movement of the drilling fluid through the pipe, is used to rotate the head of the drill bit.
In order to effectively use a mud motor a sufficient volume of drilling fluid must pass through the motor to turn it. If the volume of drilling fluid is insufficient then the motor will stop working. This makes it critical to immediately replace any drilling fluid lost to highly porous zones encountered during drilling in order to ensure continued operation of a mud motor system. Unfortunately, most horizontal directional drilling equipment has only limited ability to mix and store fresh drilling fluids. As a result, the use of mud motor-driven equipment is often difficult or impossible in highly fractured rock or in zones of large rock cobbles where significant loss of drilling fluids to the formation is common. For this reason it is particularly important that the loss of drilling fluids to the formation be controlled so that the drilling fluid can be returned to the surface where it can be cleaned and reused.
Drilling fluid technology has also been adapted for use in the excavation of earthen trenches for foundation and subterranean hydraulic barrier construction. In this application the excavation fluid is primarily used to stabilize the trench walls. When properly practiced the technique is sufficiently successful to enable excavation through unstable soils, such as loose sands, to depths of 50 to 100 feet or more. Where excavation fluid is rapidly lost from the trench into the formation the integrity of the trench walls can be compromised often leading to wall collapse.
Many attempts have been made over the years to devise methods of controlling the loss of fluids to porous subterranean formations during all types of drilling and excavation operations. The references cited here provide a listing of many of the patented technologies that have been invented for this purpose. These technologies suffer from being too complex, too difficult to use, too imprecise and difficult to properly position in the borehole so as to adequately and consistently seal the desired zone in the formation, or too prone to premature gellation causing problems in mixing, pumping or placement. This is particularly true for small drilling operations such as are typical for water, mineral exploration and environmental monitoring wells or for horizontal directional drilling. In these cases it is necessary to have a simple method of easily placing a sealant and plugging agent that will not plug the mixing, pumping or conveying equipment while still effectively sealing the porous zone in the borehole. The composition and method of the present invention succeed in doing this.
It is, therefore, the primary object of the present invention to provide an improved composition and method for the sealing and plugging of porous subterranean formations.
It is also an object of the present invention to provide a composition that is capable of controlling the loss of drilling fluids into porous or fractured subterranean formations.
It is another object of the present invention to provide a method of drilling a well into the earth whereby the phenomenon of loss of the drilling fluid into porous or fractured formations is substantially or entirely eliminated.
It is another object of the present invention to provide a composition and method that is capable of plugging and sealing porous zones within subterranean formations by introducing a plug or pill into the borehole that is useful for sealing discrete, periodic high porosity zones within a formation.
It is yet another object of the present invention to provide a composition and method that is capable of plugging and sealing frequently encountered porous zones or continuous porous zones within a formation penetrated by a well or borehole by using the composition as a drilling fluid.
It is a further object of the present invention to provide a composition and method that is capable of stabilizing boreholes, especially horizontal boreholes in unstable formations such as gravel or loose sand formations, particularly where the borehole must be temporarily abandoned or left idle, so that it can remain intact and can be re-entered at a later time and the drilling can be continued without interference from hole collapse.
It is also an object of the present invention to provide a composition that is capable of controlling the loss of excavation fluids into porous subterranean formations.
It is another object of the present invention to provide a method of excavation into the earth whereby the phenomenon of loss of excavation fluid into porous or fractured formations is substantially alleviated or eliminated entirely.
It is another object of the present invention to provide a composition and method that is capable of sealing and plugging porous zones within subterranean formations by introducing a plug or pill into excavation trench, pit or hole that is useful for sealing discrete, periodic high porosity zones within a formation.
It is yet another object of the present invention to provide a composition and method that is capable of sealing and plugging frequently encountered porous zones or continuous porous zones in a formation intersected by an excavation trench, pit or hole by using the composition as an excavation fluid.
It is a further object of the present invention to provide a composition and method that is capable of stabilizing excavated trenches, pits or holes in unstable formations such as gravel or loose sand formations, particularly where the excavation must be temporarily abandoned or left idle, so that it can remain intact and can be re-entered at a later time and the excavation can be continued without interference from wall collapse.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
The present invention is directed toward a unique composition and method useful for the sealing and plugging of zones of highly porous, subterranean formations that may be encountered during the drilling of holes using the technique of horizontal directional drilling. The invention may also be used for plugging and sealing permeable formations encountered during the drilling of vertical bore holes, such as those drilled for water, mineral exploration and environmental monitoring wells, oil and gas exploration wells or for any other type of well. The invention may further be used for sealing and plugging zones of high permeability in formations encountered during excavation of trenches using the technique known as slurry trench excavation, or any other subterranean hole, void or gap encountered during any boring or excavation in order to limit or prevent the movement of water through it.
In particular it has been found that the injection into the bore hole of a suspension comprised of water, or an aqueous, clay-based drilling fluid, mixed with granular bentonite and granular water-insoluble, water-swellable, cross-linked polyacrylamide that have been coated with a non-cross linked, water-soluble, high molecular weight polymer substantially and unexpectedly seals and plugs high permeability formations that are encountered during drilling without causing plugging of the mixing, pumping or conveying equipment or, without forming a troublesome plug in the borehole that might impede further drilling or excavation.